(1) Field of the Invention
The present invention relates to a light frequency controller wherein two lights having a stabilized frequency difference are generated using a single frequency light source, more particularly, it relates to a light source device preferable for use when a heterodyne measurement is carried out using light.
(2) Description of the Prior Art
Recently, methods have been proposed for measuring various physical quantities by using a laser light. In a measuring method using the wavelength or frequency of the laser light as a reference, the frequency of the light is high, i.e., more than 10.sup.14 Hz, and thus a direct conversion to an electric signal is impossible. Usually, the light to be measured suffers from interference from another light having a slightly different frequency from the light to be measured, causing a beat signal to be generated. The heterodyne measurement method is performed by detecting the phase or the frequency of the beat signal. In this method the magnitude of the difference of the two frequencies must be within the range of approximately 10.sup.7 to 10.sup.10 Hz (1/10.sup.4 to 1/10.sup.8 of the frequency of the light to be measured), in which range processing by an electrical circuit becomes possible, the frequencies of the two lights, respectively, are stable and the frequency difference between them is also stable.
In practice, the two lights having the above-mentioned difference of frequencies cannot be obtained from two independent laser light sources, because of their frequency stability. Therefore, when using a single laser light source, the methods of generating two lights having a constant difference of frequencies, for example, by utilizing the Zeeman effect or by utilizing a diffraction effect due to ultrasonic waves, are proposed. However, in the method utilizing the Zeeman effect, a large magnet is required to obtain a desired difference of frequencies, and since the two lights having the difference of frequencies are obtained by the separation of clockwise circular polarization light from counter-clockwise circular polarization light, this method cannot be applied to a semiconductor laser which generates a linear polarization light (one of two linear polarization lights, i.e., transverse electric (TE) wave and transverse magnetic (TM) wave).
In the method utilizing the diffraction effect due to ultrasonic waves, the ultrasonic wave modulator requires a very strong driving power to obtain a desirable difference of frequencies through a change in the Bragg reflecting condition obtained with ultrasonic waves, and countermeasures are required against the heat generated.
On the other hand, as shown in FIG. 1, a method is proposed in which a light I (frequency .omega.) from one light source is separated into two lights, and the respective lights are modulated by external signals (frequency .DELTA..omega.) having a phase deviated by 1/4 period using photoelectric elements 1a and 1b. The two lights Ia and Ib obtained by the above-mentioned procedure, which include the frequencies .omega.+.DELTA..omega. and .omega.-.DELTA..omega., are further separated into two lights. After an appropriate light path difference between the separated lights Ia and Ib is obtained, the two lights are then combined, and a further two lights Oa (frequency .omega.+.DELTA..omega.) and Ob (frequency .omega.-.DELTA..omega.) are taken out separately.
Using the above method, the two lights having a desirable difference of frequencies can be obtained by a comparatively small external signal (electric or magnetic signal). Also, the two lights having the difference of frequencies are obtained without a magnetic field, a semiconductor laser can be used as a light source, and the problems in the conventional method are effectively solved. On the other hand, when the light having the original frequency is modulated, the intensity thereof is effected by amplitude modulation. As a result, the modulated light is superimposed on the beat signal, and subsequently, separation of the superimposed light becomes difficult. The above problems in the conventional methods give rise to disadvantages in that the field of application of these methods is limited.